AvianVisitors: BirdNET-Pi collage frontend overlay

Adds avian/ on top of upstream BirdNET-Pi:
- frontend/    static collage UI (mask-packed bird tiles, sized by detection count)
- assets/      ~450 bundled kachō-e illustrations + cutouts + binary masks
- api/         PHP shims for recent/lifelist/firstseen/timeseries JSON
- scripts/     installer + Gemini pregen + editable prompt template
- caddy/       snippet mounting /collage on existing Caddy
- forwarding/  optional HA / MQTT / Cloudflare configs

Default install: http://birdnet.local/collage/ with no auth.

Upstream BirdNET-Pi README preserved at README.upstream.md.

avian/forwarding/cloudflared.yml

@@ -0,0 +1,11 @@
+# Cloudflare Tunnel config — copy to /etc/cloudflared/config.yml and
+# replace TUNNEL_UUID with the value printed by `cloudflared tunnel create`.
+tunnel: TUNNEL_UUID
+credentials-file: /etc/cloudflared/TUNNEL_UUID.json
+
+ingress:
+  # Public collage — points at the local Caddy on port 80.
+  - hostname: birds.your-domain.com
+    service: http://localhost:80
+  # Catch-all required by cloudflared
+  - service: http_status:404